1. Field of the Invention
The invention relates to a water-treating apparatus for activating silicic acid contained in water. The apparatus, when applied to, e.g., a circulating water system, effectively provides circulating water in which the generation of rust and scale is prevented.
2. Description of the Related Art
Japanese Patent Application Laid-open No. Hei 2-227193 (227193/90) discloses providing water that is resistant to generating rust and/or scale by generating a magnetic field and electronic field in water channel system and passing water through the system at a constant flow rate.
Various attempts have been made to develop an apparatus which makes it possible to minimize the generation of rust and/or scale. However, it is still in a stage of trial and error.
In natural water, silicic acid is present either in the form of a monomer or in the form of a polymer of two of more monomers. It has been known that polymers of silicic acid, with the increase in the degree of polymerization, turn into a colloidal state, and the colloidal polymers of silicic acid bind to, or enclose, various organic and/or inorganic ions forming a soluble complex salt. In this specification, the phrase "activation of silicic acid" means the conversion of silicic acid monomers into such polymers.
Heretofore, conductivity meters have been used for the control of the concentration of cooling water. This is because value of electric conductivity can be an indicator of the degree of concentration of cooling water. The density of chlorine ions has also been used as a similar indicator of concentration. However, the actual concentration patterns of cooling water are highly complex and influenced in a sensitive manner by factors such as pH, temperature, etc., as well as by the nature and interactions of various solutes (such as ions and salts) contained in cooling water. The patterns therefore could not be fully defined by theory. In such a case, statistical treatment can be the most rational scientific approach.
Based on years of experience and observation of the changes of pH, free carbon dioxide and the saturation index, the present inventors have converted the electric conductivity of supplementary water ( fresh water ), or cooling water, chlorine ions, sulphate ions, M alkalinity, total hardness, silicic acid ions, and the data obtainable by the chemical analysis of evaporation residues into data on a concentration multiple of cooling water/supplementary water. The inventors have also found that the state of cooling water, or a circulating cooling water system, can be best grasped in a realistic manner by subjecting the data on concentration multiples to statistical treatment.
To be more specific, concentration multiples of each of the above seven items are determined for a specific sample and the average concentration multiple (X) of the items is calculated therefrom. If the concentration multiple of sulphate ion is larger than the average concentration multiple X and, at the same time, the concentration multiple of M alkalinity is less than the average concentration multiple X, the data show that the sample may be influenced by SO.sub.2 -containing exhaust gas through a cooling tower. Time series-like changes of each of the concentration multiples may indicate the possibility of the formation of scales consisting of CaSO.sub.4 ( calcium sulphate ) or CaSiO.sub.2 ( calcium silicate). It is easier to determine these data quantitatively, in comparison with the case where data on chemical analysis are merely compared.
When the concentration multiples of the test items are designated as follows:
X.sub.1 =Concentration multiple of electric conductivity PA0 X.sub.2 =Concentration multiple of chlorine ion PA0 X.sub.3 =Concentration multiple of sulfate ion PA0 X.sub.4 =Concentration multiple of M alkalinity PA0 X.sub.5 =Concentration multiple of total hardness PA0 X.sub.6 =Concentration multiple of silicate ion PA0 X.sub.7 =Concentration multiple of evaporation residue
and their standard deviation (S) is calculated according to the following formula: ##EQU1## in which i=1, 2, 3, 4, 5, 6 or 7, and the coefficient of variation (CV%) is calculated in accordance with the following formula: ##EQU2## there can be obtained more accurate information on the generation of rust and scale.
To be more specific, when water of a certain quality is concentrated, the value of the standard deviation remains small up to a certain concentration multiple. However, when it is concentrated beyond a certain concentration multiple, the value of the standard deviation abruptly becomes greater, and the controlling state of the cooling water becomes worse, thus causing troubles such as the generation of scale, corrosion and slime. This is because the concentration multiple of, e.g., total hardness and silicate ion drops to a value that is smaller by a large margin than X. As a result, Ca, SiO.sub.2 and the like are deposited from the cooling water in quantities corresponding to the margin, resulting in the formation of scale. This interpretation agrees very well with the scale and corrosion actually observed when a heat-exchanger or a boiler (steam boiler) is opened.
Based on a large number of experiments, the inventors have found that when the coefficient of variation (CV %) of a cooling water is not greater than 10%, the water is in a best controlled condition, i.e., in a state where there is no generation of scale and corrosion and that when it is less than 20%, the cooling water is still in a permissible controlled condition. Thus, the coefficient of variation can be a useful standard for the control of the quantity of water. It has also been found that it is difficult to maintain the coefficient of variation at a value not greater than 20% if the concentration multiple exceeds five even in the case where a conventional scale inhibitor is employed, although the result more or less varies depending on the quality of water.
Silicic acid contained in water plays an important role in the generation of rust and scale, as is described above in relation with Japanese Patent Application Laid-open No. Hei 2-227193. Although the mechanism has not been fully defined, it is believed that the state of silica dissolved in natural water is H.sub.2 SiO.sub.3 and that it is present in an ionized state (HSiO.sub.3.sup.-). If the ionized monomers are chemically activated by gaining a large energy in some way or other, polymerization of the monomers takes place. In the course of the polymerization, they form complex salts which are soluble with metal and other ions in water, a stable suspension is formed by the peptization effect of the polymers, and a film is then formed on the surface of metal. As a result, the generation of rust, incrustation (tubercle) of rust and scale can be prevented.
The inventors have conducted extensive investigations on the method of activating silicic acid contained in water and found, quite surprisingly, that silicic acid could be effectively activated by passing silicate-containing water through an oscillating magnetic/electric field generated by linear antenna connected to a particular high-frequency power source. Under such conditions, the above-described CV% could be maintained at a value not greater than 20% without relying on cases where water was treated without any control of the concentration of silica contained therein, prior to the treatment with a magnetic/electric apparatus, and that not only the generation of rust and scale could be prevented, but the quality of the water could be improved (Japanese Patent Application Laid-open No. Hei 3-221188 (221188/91)).
In the above treatment, an apparatus is used comprising a steel pipe which is provided with an inlet and an outlet for water and in which is positioned a linear antenna of a length 1, 1/2 or 1/4 times that of the wavelength of a high frequency wave, generated by a high frequency power source of 150 to 450 MHz which is connected to the antenna used to generate an electric field. In this case, however, the flow of water passing through the pipe is not controlled and, as a result, a turbulent flow or a laminar flow parallel to the axis of the linear antenna is formed. It is therefore impossible to retain water in the electric/magnetic field for a period of time sufficient to activate the silicic acid contained in the water. Hence the generation of rust and scale could not be prevented to a sufficient degree.